The invention relates generally to the field of marine electromagnetic surveying of subsurface rock formations. More specifically, the invention relates to systems for simultaneously acquiring in-line (common transmitter and receiver dipole moment orientation) and cross-line (transmitter and receiver having orthogonally disposed dipole moments) electromagnetic signals.
Marine controlled source electromagnetic (CSEM) surveying is a geophysical surveying technique that uses electromagnetic (EM) energy to identify possible hydrocarbon bearing rock formations below the bottom of a body of water such as a lake or the ocean. In a typical marine CSEM survey, an EM source is typically towed over an area of interest in the Earth's subsurface, and a number of EM sensors are disposed on the water bottom over the area of interest to obtain signals related to the distribution of electrical resistivity in the subsurface area of interest. Such surveying is performed for a range of EM source and EM sensor positions. The EM source emits either or both a time varying electric field and a time varying magnetic field, which propagate outwardly into the overlying seawater and downwardly into the formations below the water bottom. The sensors most commonly used detect and record the induced electric field at or near the water bottom. The time varying EM field may be induced by passing electric current through an antenna. The electric current may be continuous wave and have one or more discrete frequencies. Such current passing through an antenna is used for what is referred to as “frequency domain CSEM” surveying. It is also known in the art to apply direct current to an antenna, and to produce transient EM fields by switching the current. Such switching may include, for example, switching on, switching off, inverting polarity, and inverting polarity after a switch-on or switch-off event. Such switching may be sequenced in time, for example, equally time spaced, or in a time series known as a “pseudo random binary sequence.” Such switched current is used to conduct what is referred to as a “transient CSEM” or “time domain CSEM” survey. It is also known in the art to tow the EM receivers on cables, in a manner similar to the sensor cables (“streamers”) used in a common type of marine seismic surveying.
The EM energy is rapidly attenuated in the conductive seawater, but in less conductive subsurface formations is attenuated less and propagates more efficiently. If the frequency of the EM energy is low enough, the EM energy can propagate deep into the subsurface formations. Energy “leaks” from resistive subsurface layers, e.g., a hydrocarbon-filled reservoir, back to the water bottom. When the source-sensor spacing (“offset”) is comparable to or greater than the depth of burial of the resistive layer (the depth below the water bottom), the energy reflected from the resistive layer will dominate over the transmitted energy. CSEM surveying uses the large resistivity contrast between highly resistive hydrocarbons and conductive aqueous saline fluids disposed in permeable subsurface formations to assist in identifying hydrocarbon reservoirs in the subsurface.
In an example relevant to the present invention, the sensor layout in an EM streamer system typically consists of spaced apart electrode pairs distributed along the length of the streamer. Voltage measuring circuits are associated with each of the electrode pairs, or may be switchably associated with more than one pair of such electrodes. Voltage measurements across the pairs of electrodes in response to the detected electric field amplitude are transmitted to a recording unit on the survey vessel for interpretation, or for later interpretation at a different location. The foregoing arrangement is described, for example, in U.S. Pat. No. 7,446,535 issued to Tenghamn et al.
Multiple EM field component acquisition in towed EM surveying is non-trivial due to the difficulty in measuring cross-line EM field components (i.e., components wherein the transmitter and receiver have orthogonally disposed dipole moments). Traditionally, the cross-line EM field components are omitted in EM data analysis as the main part of the information of the sub-surface structure can be found in the in-line electric field component (i.e., component wherein the transmitter and receiver share a common dipole moment orientation). However, the inability to measure cross-line EM field components reduces the uniqueness of the processed result, which is a drawback from a data analysis perspective. Cross-line electric field component acquisition can be realized, for example, through a multi vessel operation. However such a set-up is associated with significantly increased operational costs. There exists a need for multiple component EM data acquisition that avoids the complication of additional survey and/or towing vessels.